A topological insulator is a type of insulating material with conducting surface states, meaning that electrons can only move along the surface of the material. Carriers in these surface states have a spin oriented perpendicular to their velocity. Therefore, spin-up electrons travel in one direction and spin-down electrons travel in the opposite direction. The spin orientation is caused by the spin-orbit interaction. Topological insulator behavior has been observed in compounds with heavy elements having strong spin-orbit coupling, such as antimony, bismuth antimonide, bismuth selenide, bismuth telluride, antimony telluride, and bismuth antimony tellurium selenide, among other compounds.
Topological insulators possess spin polarized carrier transport at their surfaces with a spin density proportional to the current density. See Dimitrie Culcer et al., Physical Review B 82(15), 155457 (2010). Therefore, devices with tunable control of spin polarized currents might be realized, which would be useful for spintronics applications. See Dmytro Pesin and Allan H. MacDonald, Nat Mater 11(5), 409 (2012). In particular, the spin polarized properties of topological insulator surface currents may enable new detection mechanisms useful in photoconductive devices.